Clarification: stating that there currently is no explicit mention of 10th-level slots or spells is not helpful, I'm fully aware of that - it's the whole point of this question.

Therefore, unless you can provide a reference to an official source that explicitly disallows 10th-level spell slots, please refrain from answering. Note that, while they're generally not considered official, a quote from any designer (not just Crawford), will suffice.

2 Answers
2

Yes

That is what it says it does, so that's what it does. There're no serious problems with this, either, yet; the Book of Exalted Deeds is an artifact level magic item and (as your example shows) most spells just work a little better with this effect. The only potential issue comes from the ability for a spell to be written needing slots higher than 9th not to exist (since the developers will continue to expect 10th level slots to be impossible, probably), but no such content has yet been created.

\$\begingroup\$Not sure if it changes your mind, but take a look at the addition to my answer from this comment.\$\endgroup\$
– NautArchApr 5 '18 at 13:07

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\$\begingroup\$@NautArch I'm aware. I don't think any of those rules or any similar possible rules have any bearing on the question, because specific v.s. general. Also they are, generally, for spell levels, not for spell slot levels. Also the book makes things 'count as' one level higher, not actually be one level higher so the whole argument's kinda irrelevant anyways. The only thing that I've seriously considered editing the answer to address is Globe of Invulnerability as regards my last sentence, but that spell is still trivially dealt with even at 10th level, so it's not really a problem.\$\endgroup\$
– Please stop being evilApr 5 '18 at 16:50

\$\begingroup\$@NautArch You may want to address Globe of Invulnerability in your final section, though, instead of presenting free Dispel Magic and Counterspell upcasts as a problem (they're not any more than any other good spells are).\$\endgroup\$
– Please stop being evilApr 5 '18 at 16:51

\$\begingroup\$I had used those spells because to use them the DC would be different than normal (DC 20, can't upcast to make it automatic.) And fair enough on your reasoning, it did read like you were putting more on "it just doesn't matter" than consideration of 10th level spells.\$\endgroup\$
– NautArchApr 5 '18 at 16:52

\$\begingroup\$@NautArch Oh, I see, you meant that those require attention from the perspective of other people using them on a 9th level Cleric spell cast by someone with the Book! I thought you meant that either being able to upcast from 9th to 10th with those spells wasn't really good enough because the likelihood of facing someone else with 10th level effective casting is nil or that being able to boost a second level spell to take out a 3rd level spell (for example) meant that the player would be massively more efficient at countering hostile magic than before. You may want to clarify in your answer.\$\endgroup\$
– Please stop being evilApr 5 '18 at 17:13

No. You can't turn up the volume to 11.

Spell Levels are currently limited to Level 9. There is no class that has a 10th level spell slot and there are no rules that suggest there is a 10th level spell slot. Neither are there current 10th level spells that could use a 10th level spell slot.

When a character casts a spell, he or she expends a slot of that spell’s level or higher, effectively “filling” a slot with the spell. You can think of a spell slot as a groove of a certain size—small for a 1st-level slot, larger for a spell of higher level. A 1st-level spell fits into a slot of any size, but a 9th-level spell fits only in a 9th-level slot.

This clearly states that there is no 10th level (currently) spell that you can cast. While it doesn't explicitly state there is no 10th level slot, the implication is that without 10th level spells there aren't 10th level slots - otherwise there would be upscaling options allowing you to utilize that.

One can reasonably extrapolate how it'd work, but the current rules do not include 10th level and there are no other examples list going to a 10th level (such as examples that list surpassing 20 for an Ability Score.)

The following phrase from the Book description does not create a new spell slot level, it simply upcasts to one level higher. The current cap of a 9th level spell remains:

any spell slot you expend to cast a cleric or paladin spell counts as a spell slot of one level higher.

Unlike the rules around Ability score raising with the Tomes/Manuals, this book does not state that it increases spell levels beyond the current cap of 9th.

A free upcast

What this book does do is give you a free upcast without having to use a higher level slot. That's still a pretty great benefit without having to create a 10th level spell slot that just doesn't exist in the current rules.

The stated benefits are all still applicable and do not require generating an unlisted option (10th level spell slot) in order to be useful and/or work.

Interactions to consider and the Slippery Slope

If you do allow this in your games, be aware that this changes Counterspells and Dispel Magics. A simple fix, but just something to be aware of.

And for a warning, if this allows 10th level, then why couldn't someone create a 10th level scroll? Or an 11th? Once you've confirmed the existence of 10th level, then that level now exists and must be accounted for.

\$\begingroup\$the quoted snippet from the PHB mentions spells. Spells are not spell slots, and vice versa. As far as I can see, there is no RAW cap for spell slot levels, so there is no need for an explicit exception.\$\endgroup\$
– PixelMasterMar 29 '18 at 22:14

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\$\begingroup\$@PixelMaster "When a character casts a spell, he or she expends a slot of that spell’s level or higher, effectively "filling" a slot with the spell. You can think of a spell slot as a groove of a certain size - small for a 1st-level slot, larger for a spell of higher level. A 1st-level spell fits into a slot of any size, but a 9th-level spell fits only in a 9th-level slot." (PHB, emphasis mine)\$\endgroup\$
– Michał PolitowskiApr 5 '18 at 11:58

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\$\begingroup\$that's true, but they probably didn't consider that there would be an item that increases spell slot level. Anyways, this paragraph only explicitly restricts using 10th-level spell slots for 9th-level spells, which is pointless anyways - nothing states that you couldn't upcast an 8th or lower level spell with a 10th-level spell slot.\$\endgroup\$
– PixelMasterApr 5 '18 at 12:01

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\$\begingroup\$@PixelMaster We are always referencing the PHB and DMG together here and both were published in 2014. To assume that the Wizards didn't know of the DMG is kind of silly. WIth no 10th level slots listed and the limitations quoted above, I do not see how/where you can have a 10th level slot without houseruling/homebrewing it.\$\endgroup\$
– NautArchApr 5 '18 at 12:54

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\$\begingroup\$In my opinion, the intent of the quoted passage is merely to explain how a 9th level spell can't fit in a level 8 or lower slot. Judging by how 5e tends to use regular English, I get the impression that the phrase "a 9th-level spell fits only in a 9th-level slot" was only meant to explain that you can't use an 8th level slot - not to disallow using a 10th-level slot. Obviously, there generally are no 10th-level slots, and that is why they weren't considered in that paragraph - which is the whole point of this question. If they wanted to prevent 10th-level slots, they would have written it.\$\endgroup\$
– PixelMasterApr 5 '18 at 17:16